Huntington's disease (HD) is an incurable inherited neurodegenerative disorder resulting from an abnormal expansion of CAG repeats in exon 1 of the huntingtin (Htt) gene. It remains unclear why mutant Htt is toxic to neurons, especially medium spiny neurons (MSNs) that are 3-aminobutyric acid (GABA) projection neurons in the striatum. It has been widely accepted that one of the major reasons for the lack of understanding of HD pathogenesis is the paucity of reliable experimental models. Cell culture models of HD are essential for understanding the molecular mechanisms of HD and have been employed in a number of studies. Although the HD stable cell lines developed previously exhibit some of the pathological features of HD, almost none of them possess adequate neuronal markers or other neuronal properties and thus they are not ideal for modeling HD. To better understand the pathogenesis of HD, additional novel cell models of HD are required. Recent rapid advances in reprogramming human somatic cells into induced pluripotent stem (iPS) cells have made it possible to generate iPS cell models from HD patients. However, since the current HD iPS cell lines contain multiple viral inserts that are used for delivering several reprogramming factors, it is questionable whether these cells can model HD well because the multiple viral inserts may disrupt endogenous gene expression and cause abnormal cellular differentiation. To overcome this problem, we recently generated virus- and transgene-free iPS cells derived from HD patient fibroblasts and, in this project, we propose to model HD using these iPS cells. We hypothesize that these HD iPS cells, free of any exogenous DNA sequence, can be successfully induced to differentiate into mature neurons, especially GABAergic neurons, once adequate conditions are given. Additionally, we hypothesize that they will reproduce the major pathological features of HD across neuronal maturation and aging in cell culture. To test these hypotheses the following specific aims are proposed, in which two undergraduate students will participate in the studies. 1. To characterize HD patient-derived iPS cell lines free of viral or transgenic inserts. 2. To induce the iPS cells to differentiate into GABAergic neurons. 3. To characterize the GABAergic neurons derived from the HD iPS cells. PHS 398/2590 (Rev. 11/07) Page Continuation Format Page PUBLIC HEALTH RELEVANCE: Huntington's disease is an inherited neurological disorder that selectively causes nerve cells to die in the specific brain regions and effective therapy for this disease is still unavailable at present. Although various cell and animal models of Huntington's disease have been created over last decade, so far there is no ideal model that can replicate all of essential pathological features of this disease. This project is designed to determine whether the nerve cells that are converted from patient induced pluripotent stem cells can mimic major aspects of pathology of this disorder in cell culture.